1. Technical Field
Example implementations are directed to cellular systems, and more specifically, to cell selection and handover in heterogeneous networks.
2. Related Art
The Long Term Evolution (LTE)-advanced network cellular system is designed to improve the spectral efficiency by reducing the cell size via heterogeneous deployment of a diverse set of enhanced Node Bs (eNBs). In a heterogeneous network, the macro eNBs are deployed with a high transmit power and the overlaid pico eNBs are deployed with relatively lower transmit power to improve the coverage and UE throughput. Such an overlaid deployment can also provide capacity gain by increasing the spatial reuse of the spectrum.
When a user equipment (UE) is powered on in an LTE-advanced network, the UE first performs cell selection. The UE may searches for a suitable cell (which could be a macro or pico cell) to associate with. During the process of cell selection, the UE measures reference signal strength (e.g., Reference Signal Received Power/Reference Signal Received Quality (RSRP/RSRQ)) for each neighboring cell and selects a cell based on some criterion, e.g., the cell with the highest RSRP value. After the UE selects a cell, the UE verifies the cell's accessibility by reading the master information block (MIB) carried over the physical broadcasting channel (PBCH). After the UE succeeds to verify the selected cell's accessibility, the UE will associate itself to the selected cell. If the UE fails to read the MIB of the selected cell, the UE will discard the selected cell and repeat the above procedure until the UE finds a suitable cell to associate with. After the UE is associated to a cell, its status becomes RRC_IDLE. Note that the UE in RRC_IDLE status may associate itself to another cell following the above cell selection/reselection procedure when it moves into a new area. If there are a large number of pico cells deployed in an area, a RRC_IDLE UE will perform frequent cell selection/reselection when it is moving, which may reduce battery life.
When a UE has data packets to transmit or receive, the status of the UE becomes RRC_CONNECTED. An eNB can handover a RRC_CONNECTED UE to another eNB based on the RSRP/RSRQ measurement information reported by the UE. Similar to the RRC_IDLE case, if there are a large number of pico cells deployed in an area, frequent handover may occur which may increase the probability of dropping calls.